Photoreversible, organic compounds are especially attractive materials for use as molecular switches. Photoreversible compounds are stimulated by light, i.e., they provide reversible switching processes based on photochemically induced interconversions. Photochromism, a reversible change induced by light irradiation between two states of a molecule having different electromagnetic absorption spectra, is commonly associated with such photoreversible systems. Photochromic switching processes are typically based on photocyclization of isomers, the conversion of olefinic (cis/trans) isomers, photoinduced electron transfer, and keto-enol tautomerism.
Photochromic molecular switches based on the trans→cis isomerization of azobenzene are useful for many purposes, including industrial dyes, actuators, nonlinear optical devices, liquid crystals, molecular machines, ion channel modulators, and the like. Ultraviolet (UV) light often is used to induce the trans→cis isomerization. However, UV light can be harmful in certain applications, especially those involving in vivo systems. Hence, there is a significant need in the art for photochromic molecular switches that can be toggled between cis and trans states using lower energy electromagnetic irradiation with less scattering and that can penetrate tissues more easily (i.e., red and NIR light).